The invention relates to an arrangement for digitizing television signals, which signals comprise a luminance portion whose amplitude changes at a relatively low frequency and a high frequency color subcarrier portion, said arrangement comprising a first and second analog to digital converter, means for separating the luminance and color subcarrier portions, means for applying the separated luminance portion to the first analog to digital converter, and means for applying the separated subcarrier portion to the second analog to digital converter. The invention further relates to an arrangement for measuring television signals and to a television transmitter.
Broadcasting authorities generally require their transmitters to meet certain requirements concerning the quality of the transmitted signals. Among the more important parameters that have to be monitored or controlled are linearity and differential gain. With the growing use of computers for analyzing signals it becomes desirable to digitize the television signals to make them convenient to handle. In the United Kingdom (UK) both the British Broadcasting Corporation and the Independent Broadcasting Authority insert test signals into certain line of the television signal during the field blanking period. These signals include a portion which includes a luminance portion having a staircase form on each step of which a constant amplitude color subcarrier wave is superimposed. This can be used to measure three important non-linear distortions, i.e. luminance non-linearity, differential gain, and differential phase. If this signal is converted directly to a digital representation then in order to satisfy the Nyquist Criterion it must be sampled at a rate greater than twice the video signal bandwidth, i.e. faster than 11 MHz in the UK. In practice the analysis is easier to carry out if the sampling rate is four times the color subcarrier frequency ( 17.7 MHz). The digitization must also be very accurate since any error in the conversion process will cause a much larger error in the calculated values of luminance non-linearity and differential gain. If these parameters need to be measured to within about 1% then a 10 bit analog to digital converter is required. The combination of high speed of operation and high accuracy is expensive.
There are a variety of methods which allow the signal to be digitized in a less expensive manner, but all the known methods tend to degrade the speed, accuracy or susceptability to noise of the arrangement. One method, which is used in the Tektronix Automatic Video Measurement Set 1980 Answer, is to use a high speed reduced resolution analog to digital converter and the add a dither signal to the analog video input. The full resolution is eventually obtained by averaging the resultant digital signal over many repeated occurances of the test signal. Another method, which is used in the Thomson-CSF TTV 8400 System, is to use a low speed full resolution analog to digital converter and to build up a complete replica of the test signal by taking a few samples from each of successive test signals. This is however a slow process which is susceptable to noise.